The present invention relates in general to airfoil design and in particular to a new and useful arrangement for decreasing airfoil drag at transonic speeds using a porous surface area on the airfoil.
As the airstream passing an airfoil reaches the speed of sound, various phenomenon occur. The study of such phenomenon are important not only for supersonic aircraft but also for airfoils, such as helicopter rotor tips, which may operate at transonic speeds.
During transonic flight, a dramatic increase in wing drag takes place as the drag divergence Mach number is reached. A shock wave is formed on the airfoil at a point usually more than 50% of the chord length back from the leading edge of the airfoil. The shock wave forms a trailing boundary for a supersonic "bubble" which is totally embedded in a subsonic flow. The shock wave produces wave drag and soon after the appearance of the shock wave, the drag increases rapidly with increasing freestream Mach number leading to a "drag rise Mach number". One of the main objectives of designing a wing for transonic speeds is to obtain as high a "drag rise Mach number" as possible, subject to certain constraints. In principle, so called super critical airfoils are shaped to delay the drag rise associated with energy losses caused by shock waves and flow separation. Two articles which discuss this and are incorporated here by reference are "Low-Speed Aerodynamic Characteristics of a 14% Thick NASA Phase 2 Supercritical Airfoil Designed for Lift Coefficient of 0.7," NASA TM 81912, December 1980, by Harris et al and "Aerodynamic Characteristics of a 14% Thick NASA Supercritical Airfoil 33 Designed for a Normal Force Coefficient of 0.7," NASA TM X-72712, February 1981, by Harris.
It is also known to utilize porous surfaces on airfoils for transonic speed drag control as disclosed in U.S. Pat. No. 2,643,832 to Thwaites, U.S. Pat. No. 3,843,341 to Dannenberg et al and U.S. Pat. No. 3,128,973 to Dannenberg.